This invention relates generally to methods for fabricating rotor shafts, and more specifically to methods for repairing damaged rotor shafts for turbine generators.
At least some known turbine generators include one or more turbines coupled to a generator. The generator may include a rotor shaft that is coupled to one or more turbines via a coupling. Over time, such rotors may develop cracks, such as, but not limited to, at holes, fillets, and/or diameter changes along the shaft. Such cracks may sometimes propagate to a bore of the rotor, which may adversely affect a performance of the rotor and/or over time may shorten the useful life of the rotor. For example, the rotor shaft may include one or more keyways to facilitate radial alignment with the coupling. Stress cracks may develop at and/or adjacent such keyways, which may cause the rotor shaft to fail adjacent and/or underneath the coupling.
To repair a cracked rotor shaft, a section of the rotor shaft including the crack is sometimes cut off from an undamaged section of the rotor shaft and replaced with a new section. For example, if one or more keyways within an end of the rotor shaft that is coupled to the turbines are cracked, the cracked end may be cut from the rotor shaft and replaced with a new stub shaft end. At least some known methods for welding a new shaft section to an existing rotor shaft include using a combination of Gas Tungsten Arc Welding (GTAW) and Submerged Arc Welding (SAW). More specifically, and for example, at least some known methods include using a weld preparation angle of about 45° or more included, and applying one or more buttering layers using GTAW to trap elements in the base material and to minimize dilution into the weld. Such methods also use SAW to complete the weld between the new shaft section and the existing rotor shaft. However, the weld preparation angle and applied buttering layers may increase an amount of metal used to complete the weld, and may possibly increase a cost and/or time of completing the repair. Moreover, the use of both GTAW and SAW processes may require more equipment and/or operator training, and/or or may increase a time of completing the repair.
At least some other known methods used for welding a new shaft section to an existing rotor shaft include narrow groove GTAW, wherein a narrow groove with parallel sidewalls is used for the weld preparation. Although narrow groove GTAW may not require buttering layers to trap elements in the base material and minimize dilution into the weld, narrow groove GTAW may be limited by the depth of weld attainable. Moreover, narrow groove GTAW may require more operator training and/or specialized equipment, which may increase a cost and/or time of completing the repair.